1. Field of the Invention
The present invention relates generally to a wireless communication system, and more particularly to a planar antenna for use in an ultra-wideband wireless communication system having a frequency notch function.
2. Description of the Related Art
Currently, wideband communication systems using electric pulses have been mainly used in military applications, and even when used in non-military applications their use has been limited to detecting mines buried under the ground or searching for survivors buried under collapsed buildings. However, according to an approval given in 2002 by the Federal Communications Commission (FCC), a frequency band of 3.1 GHz to 10.6 GHz is available for industrial use in the fields of radar, position tracking, and data transmission. Therefore, ultra-wideband (UWB) systems operating in the frequency band of 3.1 GHz to 10.6 GHz are in development.
One of the most important essential components of the UWB systems is the antenna. Because the UWB systems communicate using pulses, they require specific antennas, which operate independent of frequency, and have input impedance characteristics satisfying a required wideband. Further, when such antennas are used with mobile communication equipment, due to the nature of such portable equipment, they need to be much smaller and lighter, and are preferably planar antennas, which are constructed using printed circuit board methods. Because the planar antennas can be mass-produced by using the printed circuit board methods, they are very suitable for the manufacture of communication equipment from an economic point of view.
UWB systems should not exert any effects upon existing communication systems, or disturb communication between the existing systems. In order to restrict interference with electromagnetic waves generated by existing systems, there is a need for ultra-wideband (UWB) antennas having a frequency notch function.
The kinds of antennas known to date can be basically classified into resonant antennas, and traveling wave antennas. Among the traveling wave antennas, especially, in consideration of the fact that the UWB systems require antennas that operate independent of frequency due to the nature thereof, there is a transverse electromagnetic (TEM) horn antenna, a biconical antenna, a bowtie antenna, a tapered slot antenna, etc. The TEM horn antenna and biconical antenna, however, are unsuitable for use in small wireless communication ultra-wideband systems since they are relatively large, and have a three-dimensional design. The bowtie antenna and tapered slot antenna, which are both small in size, have difficulty satisfying impedance characteristics throughout a required wideband of the wireless communication ultra-wideband systems. Therefore, novel two-dimensional small planar antennas have been recently developed.
As examples of ultra-wideband, planar antennas proposed to date, there is an antenna having two elliptical radiators (as disclosed in International Patent Application No. WO 02093690 A1), an antenna having an inverted triangular radiator structure (as disclosed in U.S. Pat. No. 5,828,340), and an antenna having leaf-shaped slot radiators (as disclosed in U.S. Pat. No. 6,091,374). These small planar antennas emphasize thorough coverage of a required wide frequency band, but do not have a frequency notch function required of UWB antennas.
A frequency band assigned to the UWB systems is in the range of 3.1 GHz to 10.6 GHz. Within this frequency band, the UWB systems require a frequency band gap between 5.15 GHz and 5.35 GHz, which is assigned to a present wireless local area network (WLAN), in order to prevent interference with electromagnetic waves generated by existing WLAN systems. Therefore, there remains a need to develop UWB antennas having a frequency notch function.